be committing a sin in murdering a family member.

2. Duncan is his king, meaning that he will be committing regicide, an extreme act of public treachery.

3. He is Duncan's host, sworn to protect his guests against harm, and therefore certainly obliged not to murder them.

4. Duncan is an excellent king. There is no altruistic reason to kill and replace him, only personal ambition.

By the time Lady Macbeth enters, he has decided not to kill Duncan, and adds two further reasons in his ensuing argument with her.

5. Macbeth has just been made Thane of Cawdor, and is at the height of his favor and success. He (and his wife) should enjoy these honors and accomplishments rather than immediately embarking on a crime which could ruin everything.

6. This attempt to kill the king might fail. What then?

All these reasons are perfectly sensible, but Lady Macbeth sweeps them aside with the sheer force of her personality, leaving Macbeth as determined to kill Duncan at the end of the scene as he was doubtful about doing so at the beginning.

Macbeth provides several valid reasons as to why he should not commit regicide during his soliloquy in act 1, scene 7. Macbeth begins his soliloquy by stating that he would certainly assassinate King Duncan if there were no consequences attached to his actions. However, he rightfully acknowledges that he will pay a heavy price for the bloody deed by saying,

But in these cases
We still have judgment here, that we but teach
Bloody instructions, which, being taught, return
To plague th’ inventor (1.7.7–10).

Suffering the consequences of his actions and jeopardizing his own well-being is the first reason Macbeth gives for not assassinating the king. Macbeth goes on to comment that he is King Duncan's kinsman, subject, and host. As a relative, subject, and host, Macbeth recognizes that it is his duty to protect Duncan from harm instead of plotting his death. By assassinating the king, Macbeth knows that he will be violating the most sacred obligations and committing the worst crime possible.

In addition to commenting on his close relationship with King Duncan and sacred duties towards him, he notes that King Duncan is a virtuous, humble leader who does not deserve to die. According to Macbeth,

... Duncan
Hath borne his faculties so meek, hath been
So clear in his great office, that his virtues
Will plead like angels, trumpet-tongued, against
The deep damnation of his taking-off (1.7.17–20).

When Lady Macbeth enters the scene, Macbeth tells her that he will not follow through with the plan, because Duncan has honored him as of late and he has "bought / Golden opinions from all sorts of people." Lady Macbeth proceeds to criticize his manhood, and Macbeth provides yet another reason for not committing regicide by asking, "If we should fail?" (1.7.65). Macbeth worries that their murderous plot will be discovered and they will fail to get away with murder.

Despite the many reasons Macbeth provides for not killing Duncan, he succumbs to his ambition and follows his wife's bloody plans.

Macbeth worries about getting caught, feels Duncan has not done anything to deserve being killed, and believes a host should not kill a guest.

The first problem Macbeth has is that killing a king is very serious. He tries to talk himself out of it in his soliloquy. He begins by saying that, if he does kill Duncan, then he needs to do it quickly. Killing the king is dangerous. Macbeth is worried about the effects it will have on the kingdom. Will one murder create a domino effect of murders?

There are additional problems because the king is Macbeth's kinsman and his guest.

He's here in double trust;
First, as I am his kinsman and his subject,
Strong both against the deed; then, as his host,
Who should against his murderer shut the door,
Not bear the knife myself (Act I, Scene 7). 

Basically, Macbeth would be violating every rule of gracious hosting by killing Duncan while he is staying at his home. You are supposed to protect your guests, not kill them. This is even worse when your guest is also your kinsman. 

Macbeth then goes on to praise Duncan, saying he is great and doesn’t deserve to die.

Besides, this Duncan

Hath borne his faculties so meek, hath been

So clear in his great office, that his virtues

Will plead like angels, trumpet-tongued, against

The deep damnation of his taking-off (Act I, Scene 7).

Duncan has been a noble and virtuous leader. People will sing his praises when he dies. He doesn’t deserve death. It sounds like Macbeth is no ethical match for him. Therefore, Macbeth worries about killing and serving as the replacement for such an excellent leader. Macbeth's only qualification for being king is that he wants to be king badly enough to kill the current king.

In act 1, scene 7, Macbeth gives a soliloquy, where he outlines several reasons for not assassinating King Duncan. The first reason Macbeth gives for not murdering King Duncan is that the king's murder will only result in more bloodshed. Macbeth acknowledges that killing Duncan will not be the end of the affair and more bloodshed will follow. He fears that his bloody deed will come back to haunt him and motivate others to follow in his footsteps. Macbeth says,

"If the assassination Could trammel up the consequence, and catch With his surcease success; that but this blow Might be the be-all and the end-all here, But here, upon this bank and shoal of time, We’d jump the life to come. But in these cases We still have judgment here, that we but teach Bloody instructions, which, being taught, return To plague th' inventor: this even-handed justice Commends the ingredients of our poisoned chalice To our own lips" (Shakespeare, 1.7.2-11).

The second reason Macbeth gives for not killing King Duncan concerns the fact that he is the king's host and Duncan is his kinsman. Macbeth acknowledges that it is wrong to murder a relative and would be considered a great offense to harm one's guest. Macbeth mentions that a host should shut the door to prevent murderers instead of committing crimes against their guests. Despite Macbeth's reservations, his wife persuades him into assassinating King Duncan, which leads to more bloodshed and the corruption of Macbeth's soul.

Macbeth hesitates to go down the path of murder because, in general, violent deeds “return / To plague th’inventor.” In other words, in murdering any person, Macbeth will bring trouble to himself.

But more specifically, Macbeth offers two reasons for not killing Duncan. First, Duncan is staying under his roof, and under the rules of hospitality in that culture, Macbeth, as a host, is expected to protect his guests. Clearly, murdering one's guest is the opposite of protection. Further, as his subject, Macbeth owes a special allegiance to his king.

Second, Macbeth acknowledges that Duncan has been a good and "meek" king, by which he means Duncan has been a just ruler. In fact, Duncan has been just and good to Macbeth himself, making him Thane of Cawdor. How can Macbeth betray such a worthy ruler and kill him based on something as insubstantial as personal ambition?

It is likely that had Lady Macbeth not intervened, Macbeth might have dropped his murderous plan.

In act 1, scene 3, Macbeth receives the seemingly favorable prophecies from the Three Witches who call him Thane of Cawdor and future King of Scotland. Immediately after the first prophecy is fulfilled, Macbeth begins to entertain the idea of murdering the king. However, Macbeth is not fully convinced that he must take action and hopes that "chance" will once again intervene, allowing him to usurp power without dooming his soul. Unfortunately, Macbeth's wife is also ambitious but significantly more resolute, callous, and bloodthirsty. Lady Macbeth proceeds to mastermind a foolproof plan and is willing to manipulate her husband into committing regicide.

In act 1, scene 7, during a soliloquy, Macbeth expresses numerous reasons as to why he is hesitant to assassinate King Duncan. Macbeth begins his soliloquy by acknowledging that his bloody actions will come back "to plague the inventor" and eventually cause him harm. Macbeth then mentions that he is Duncan's kinsman, subject, and host, which means that he should be protecting Duncan from murderers instead of plotting his death. In addition to those valid reasons, Macbeth also recognizes that Duncan is a humble, virtuous leader, who does not deserve to die.

At the end of his moving soliloquy, Macbeth acknowledges that his "vaulting ambition" is his only motivating factor to commit regicide and decides not to kill the king. Macbeth proceeds to tell his wife that he will not follow through with their bloody plan because he has been recently honored and earned "golden opinions from all sorts of people." Despite Macbeth's numerous valid reasons for not committing regicide, Lady Macbeth insults his masculinity and convinces him to cast aside his doubts. Tragically, Macbeth is persuaded into assassinating King Duncan, which directly leads to his demise.

Macbeth considers all the virtues Duncan has and the favors that the king has bestowed upon him, as well as the fact that he is related to the king and that Duncan is a guest in his castle. He also thinks about the repercussions his ghastly deed would have if he should follow through, such as the general outcry that will arise from such a malevolent act. He will be committing regicide and will be responsible for the death of a much-loved cousin and liege. There will be an outcry for justice and revenge.

Furthermore, a host is also responsible for taking care of the safety of his guests, not bringing them to harm! All these considerations are questions of morality and are disturbing to him. As such, they dwell on his mind and torment him. In his soliloquy in Act l, Scene 5, he provides all the reasons for not committing the act:

But in these cases
We still have judgment here; that we but teach
Bloody instructions, which, being taught, return
To plague the inventor: this even-handed justice
Commends the ingredients of our poison'd chalice
To our own lips. He's here in double trust;
First, as I am his kinsman and his subject,
Strong both against the deed; then, as his host,
Who should against his murderer shut the door,
Not bear the knife myself. Besides, this Duncan
Hath borne his faculties so meek, hath been
So clear in his great office, that his virtues
Will plead like angels, trumpet-tongued, against
The deep damnation of his taking-off;
And pity, like a naked new-born babe,
Striding the blast, or heaven's cherubim, horsed
Upon the sightless couriers of the air,
Shall blow the horrid deed in every eye,
That tears shall drown the wind. I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself
And falls on the other.

Not only does he hesitate, but he goes so far as to tell Lady Macbeth that "we will proceed no further in this business," meaning he has decided not to go through with their pernicious plot. She, however, persuades him not to be a coward and push through and, in the end, Macbeth is persuaded by her emotive language.

Macbeth hesitates to kill Duncan because he has a change of mind. He begins thinking about the fact that Duncan has recently honored him by promoting him to the position of Thane of Cawdor. Macbeth explains to Lady Macbeth that he has decided not to kill King Duncan. In fact, Macbeth is quite adamant about his decision to not kill King Duncan:       

We will proceed no further in this business.
He has recently honored me, and I now have the
Golden opinions from all sorts of people,
Which I want to enjoy for a bit longer, and
Not cast them aside so soon.

Macbeth seems content in being honored as Thane of Cawdor at this point. He is satisfied by having the golden opinions of all sorts of people. He admits that he does not desire to cast aside his his enjoyment so soon.

Of course, Macbeth's decision to not kill King Duncan is overruled by Lady Macbeth. She insults Macbeth's manhood. She calls him a coward. She manipulates Macbeth and insists that he follow through with the murder.

Macbeth is greatly influeneced by Lady Macbeth. He gives in to her manipulation. Macbeth becomes convinced to murder King Duncan. His actions are controlled by Lady Macbeth. She has more influence on him than he has upon himself.      

Macbeth says that he should not kill Duncan because he is his guest and kinsman.

Macbeth has wanted to be king ever since he heard the witches’ prophecies.  He also wrote a letter to his wife telling her about the witches’ predictions that he would become king, so she is on board and ready to help him dispatch Duncan if he can’t become king by legal means.

Lady Macbeth fears that her husband is too full of “the milk of human kindness” to do what needs to be done and kill Duncan.  She may be right.  Almost as soon as Macbeth returns home he has second thoughts.  Macbeth goes down the list of reasons why he should not kill the king.  First he worries about the consequences and then he comes up with reasons not to do it.

Macbeth notes that Duncan trusts him, because they are related and because he is supposedly one of the king’s loyal subjects.  He should therefore be a good host, protecting the king rather than killing him.

He's here in double trust;
First, as I am his kinsman and his subject,
Strong both against the deed; then, as his host,
Who should against his murderer shut the door,
Not bear the knife myself. (Act 1, Scene 7)

Macbeth also notes that Duncan is a good king, and does not deserve to die.

Besides, this Duncan
Hath borne his faculties so meek, hath been
So clear in his great office, that his virtues
Will plead like angels, trumpet-tongued, against
The deep damnation of his taking-off … (Act 1, Scene 7)

If Duncan were a tyrant or had mistreated Macbeth, then it might be easier to kill him.  Instead, Macbeth feels that it would be immoral to kill a virtuous king for no reason just because he wanted to be king himself.

In this speech, Macbeth makes a great change from his fiery aside when he found out Malcolm was Duncan’s heir and called Malcolm “a step/On which I must fall down, or else o'erleap,/For in my way it lies” (Act 1, Scene 4).  At that point he seemed ready to do whatever it took to become king.  Once Duncan was actually in his house, however, he found betraying him more difficult.

The reason that Lady Macbeth gives for not killing Duncan herself is the fact that she is a woman. Lady Macbeth was a very ambitious person for herself and her husband, however, it was her husband (according to what is expected of her time and place in society) that should commit the act. Therefore, Lady Macbeth questioned her husband manhood when she suggested this and may have used this as a tactical point to encourage him to commit the act.

It would be a type of Shakespearian reversed psychology, like for example when a woman tries to convince a man to do something that takes a lot of (literally) testosterone to dare to do and then when the man reasons that this is not the way, the woman decides that she may have the testosternone for the both of them together and, if she were a man, she would do it herself.

It's a bit of a castrating message from a wife to a husband not to mention the severity of it.

FIRST WITCH. All hail, Macbeth! hail to thee, Thane of Glamis!

SECOND WITCH. All hail, Macbeth! hail to thee, Thane of Cawdor!

THIRD WITCH. All hail, Macbeth, that shalt be King hereafter (1.3.50-53)

Macbeth replies that he is already Thane of Glamis. Ross soon arrives from King Duncan to praise Macbeth for his victory over Macdonwald and to tell Macbeth that the King has declared him Thane of Cawdor. The former Thane of Cawdor turned out to be a traitor (coincidentally, so does the new Thane of Cawdor), so the King condemned him to death, and decided to give Cawdor's title and property to Macbeth for his faithful service.

ANGUS. Who was the Thane lives yet,
But under heavy judgement bears that life
Which he deserves to lose. ...
But treasons capital, confess'd and proved,
Have overthrown him. (1.3.116-123)

At first, Macbeth had some doubts about what the Witches told him, but now he's starting to believe them. What the Witches said about being King also stirred up some long-repressed ambitions in Macbeth, as well as some ideas about killing Duncan.

MACBETH. [Aside.] Two truths are told,
As happy prologues to the swelling act
Of the imperial theme!...why do I yield to that suggestion
Whose horrid image doth unfix my hair
And make my seated heart knock at my ribs,
Against the use of nature? Present fears
Are less than horrible imaginings... (1.3.138-149)

Macbeth thinks about it a little more, and he has a really sensible idea—his first reason not to kill Duncan.

MACBETH. [Aside.] If chance will have me king, why, chance
may crown me
Without my stir. (1.3.155-157)

After all, Macbeth didn't have to do anything to become Thane of Cawdor. Why does he necessarily have to do anything to become King?

Macbeth might have stopped thinking about what he might do to become King right there, but why didn't he? It's because in the next scene, Duncan seals his own fate.

In act 1, scene 4, Duncan puts an obstacle in Macbeth's way to being King by declaring his son, Malcolm, the Prince of Cumberland, and heir to his throne.

Macbeth decides that the throne isn't going to come to him simply by chance or because of some scraggly old witch's prophecy. If he wants to be King, he's going to have to do something to become King, and that means he's going to have to kill Malcolm, Duncan, or both of them.

MACBETH. [Aside.] The Prince of Cumberland! That is a step
On which I must fall down, or else o'erleap,
For in my way it lies. Stars, hide your fires;
Let not light see my black and deep desires:
The eye wink at the hand; yet let that be
Which the eye fears, when it is done, to see. (1.4.55-60)

Macbeth gets caught up in the prophecy and with the idea of becoming King, and, with some urging from Lady Macbeth, he agrees to kill Duncan.

In act 1, scene 7, with Duncan under his roof, and everybody having a nice dinner, Macbeth steps out of the dining room and takes a moment to think about what he's agreed to do from practical, political, and even moral frames of reference.

Other Educators have discussed the specific reasons that Macbeth raises for not wanting to kill Duncan in his "If it were done when ’tis done, then ’twere well / It were done quickly" soliloquy.

Macbeth's concerns seem to be that he might suffer serious political consequences if he kills Duncan—that what he does will come back to "plague the inventor" (1.7.10)—and that he doesn't want to lose his honors and the good will of the people that he's worked so hard to achieve.

MACBETH. We will proceed no further in this business:
He hath honor'd me of late, and I have bought
Golden opinions from all sorts of people,
Which would be worn now in their newest gloss,
Not cast aside so soon. (1.7.34-38)

These are secondary concerns, of course. The real reason that Macbeth is reluctant to kill Duncan is what he blurts out to Lady Macbeth when he runs out of excuses about "I dare do all that may become a man..." (1.7.51).

MACBETH. If we should fail? (1.7.66)

Lady Macbeth almost laughs at him.

LADY MACBETH. We fail?
But screw your courage to the sticking-place,
And we'll not fail. (1.7.67-69)

Macbeth decides that if Lady Macbeth is strong enough and determined enough to go through with this, then so is he.

MACBETH. I am settled, and bend up
Each corporal agent to this terrible feat. (1.7.90-91)

In act one, scene seven, Macbeth contemplates assassinating King Duncan and offers several reasons why he should not commit regicide during his insightful soliloquy. At the beginning of his soliloquy, Macbeth mentions that killing King Duncan will only lead to more bloodshed, and the king’s murder will not be the "be-all and the end-all." He goes on to reason that assassinating King Duncan will only influence others to commit violent crimes, which will come back to "plague th' inventor." Macbeth is apparently concerned about his well-being following his crime and wishes to avoid Duncan’s fate.

Macbeth then offers two more reasons for not assassinating King Duncan. He says that he should not murder King Duncan because he is the king’s kinsman and host. As Duncan's kinsman, Macbeth should do his best to protect Duncan from adversaries, and as the king's host, he should close the door on murderers’s faces. Macbeth also reasons that King Duncan is such a benevolent, gracious leader that murdering him would not be justified and heaven would surely punish whoever committed the crime. After Macbeth offers several moving reasons to not follow through with the murder, he admits that his "vaulting ambition" is the only thing motivating him to assassinate King Duncan.

Having been intrigued and enticed by what the three weird sisters told him, Macbeth has written to his wife of his being made Thane of Cawdor as well as their prediction that he will be king. But, while he is excited at the prospect of being king, Macbeth is also disturbed by the workings of the supernatural world in which "nothing is /But what is not" (1.3.) and these "new honors" disturb him. Later, in Scene 7 in his soliloquy, Macbeth deliberates about killing Duncan:

If it were done when 'tis done, then 'twere well
It were done quickly....
                            but this blow
Might be the be-all and end-all--here,
But, here, upon this bank and shoal of time,
We'd jump the life to come.... (1.7.1-6)

Macbeth is concerned about several things:

• Regicide is a serious crime because murdering the ruler of the country is a grievous offense.
• Regicide also upsets the Chain of Being that the Elizabethans believed in, a chain in which the king was God's deputy on earth. So, the unlawful death of a king causes a disruption in the world of nature, and can bring plagues, famine, and unnatural activity among animals.
• It was considered important to know one's place in the Chain of Being and not try to rise above it because of ambition, which was unholy.
• King Duncan is a kinsman of Macbeth's and so Macbeth is reluctant to kill him.
• Macbeth is host to King Duncan and should be preventing any harm to the king, not harming the king himself.
• King Duncan is virtuous and heaven will surely punish him who kills this king.

For all these reasons, Macbeth hesitates about murdering Duncan; finally, he concludes that it is only his "vaulting ambition" that motivates him.

Although Macbeth's ambition is to become king, he has a moment of reason and good sense in which he realizes it would be a bad idea to murder Duncan.

First, Duncan is his cousin and the anointed king, chosen by God. Further, Macbeth knows he would be killing Duncan under his own roof. Under the laws of hospitality at that time, he is supposed to be doing all that he can to protect his guests. In other words, killing the king would violate any number of social norms.

Second, Duncan is a good king, and he has just given Macbeth a new title as thane of Cawdor. It seems especially heinous to kill such a good man who trusts him and has treated him so well.

Third, Macbeth is well aware that once he starts down this bloody path, there will be no end to it. He thinks that if he could be sure it would only be Duncan he would have to kill, that might be acceptable; however, his better self knows he will be mired in blood once he begins.

Macbeth is reluctant to kill King Duncan for a number of reasons.  First, he is Duncan's relative and his subject.  Macbeth's duty is to love and defend his kinsman and his king.  Next, he is Duncan's host, who should, he says, "against [Duncan's] murderer shut the door" and not carry the murder weapon himself (1.7.15).  Next, he says, Duncan is an incredibly good person and king; he has been a humble leader, so untainted by any kind of corruption, that the loss all of his great virtues will be too upsetting to everyone in the kingdom and the tears of Duncan's subjects will flow too much. 

Earlier in this same soliloquy, Macbeth also mentions the idea that there will be terrible consequences for himself and his soul if he commits this awful act.  He wishes that it were possible to simply commit the murder and not have to deal with any such consequences.  Further, he fears that if he kills the king and proves that such a thing can be done, others could come and do the same thing to him.

An important scene, Scene 7 of Act I of Shakespeare's Macbeth presents what is the tragic flaw in a man who has the potential to be noble if he would but listen to his conscience.  For, in this scene, Macbeth struggles with his conscience in an internal conflict between his "vaulting ambition" and his blood ties and loyalty as a Scottish nobleman to his king, Duncan as well as his fear of eternal damnation.

In his soliloquy while preparations are being made in his castle for the honored dinner guest, King Duncan,  Macbeth considers the argument against his murdering his king:

First, as I am his kinsman and his subject

....then, as his host,

Who should against his murderer shut the door,

Not bear the knife myself.

Then, Macbeth reflects that Duncan, besides being his relative and king, is a virtuous man:

...Besides, this Duncan

Hath borne his faculties so meek, hath been

So clear in his great office, that his virtues

Will plead like angels...

And, as such a virtuous man, if Duncan is killed, surely the murderer risks damnation:

The deep damnation of his taking-off....

Realizing that he has no reason, no just motive for killing Duncan, Macbeth recognizes that it is only his desire for power that overrides all the other reasons: 

Valuting ambition, which o'erleaps itself

And falls on th' other (1.7.13-28)

Thus, Macbeth's concession to his cupidity over his conscience is his tragic mistake, for it effects his later demise.

First, Macbeth says, "If it were done when 'tis done, then 'twere well / It were done quickly."  In other words, if simply killing Duncan would conclude the whole affair and render Macbeth king, then he would go ahead and do it now.  However, a lot will have to happen in between the murder and his coronation: he will have to lie to his friends, act as though he is aggrieved and shocked by the crime, and perhaps even engage in some political jostling with Duncan's sons.

Next, he expresses his knowledge that the consequences of this murder will have to be resolved. The murder itself will not neatly conclude by itself; otherwise, he would be willing to risk everything, including his soul's time in the afterlife, to get the murder finished.

Then, he suggests that he will "teach / Bloody instructions" that could "return / To plague th' inventor."  What he means is that, in committing such a violent crime, he will inadvertently show others that one can commit such a crime and get away with it.  If he kills Duncan, then it stands to reason that someone could turn around and kill him next!

Next, he considers that he is both Duncan's "kinsman," or family member, and "subject," as the king ranks above him.  Further, he is Duncan's "host," which means that he should be fighting to keep anyone wishing Duncan harm at bay, not considering murder himself.  

After this, he reflects on Duncan's virtues and his goodness as a king, saying that he has been "So clear in his great office."  Duncan has been humble and honest, so people everywhere will grieve for his loss.

Macbeth realizes that killing the king will have several consequences. He realizes that his soul will be damned for the murder and he will pay a very harsh price in the afterlife. In addition, the murder of the king will only cause more violence, some of which will be directed at Macbeth because one, he is the murderer, and two, he would now be the king.
In addition to these consequences, Macbeth recognizes that Duncan has been a good king and has shown Macbeth nothing but respect. In fact, Duncan has just rewarded Macbeth with the title of Thane of Cawdor. Macbeth realizes that the only reason he has to kill the king is his own ambition. So, as a subject of the king and as his host while Duncan visits, Macbeth should protect the king from harm, not do harm to him.

Macbeth says that he should not kill Duncan because they are related, because Duncan is his king, and because he (Duncan) is Macbeth's guest, and as host Macbeth should protect him. He also says that there will be justice against such an action, as well as simple revenge. (Someone might kill him the same way.) Since Duncan is in this situation good and innocent, even angels might cry out against the crime. Everyone will find out, Macbeth says, and what makes it worse is that he doesn't really have a valid reason for killing Duncan.

There are many reasons that Macbeth reviews in his mind and in discussion with Lady Macbeth as to why he should give up the plan to kill Duncan. As has been mentioned, these include the personal excellence of Duncan, Duncan's trust in him, and Duncan's position at the moment as a guest in Macbeth's castle. However, the most important reason -- even more important than the prospect of eternal damnation for such a murder -- is given by Macbeth in Act I, Scene 7:

But in these cases
We still have judgement here, that we but teach
Bloody instructions, which being taught return
To plague the inventor. This even-handed justice
Commends the ingredients of our poison'd chalice
To our own lips.

In other words, if Macbeth murders Duncan, he establishes a precedent that will come back to haunt him: his deed will "plague the inventor." How can he ask anyone to be loyal to him if he has gained the throne by disloyalty and so "taught" the "bloody instructions" that a superior is to be stabbed in the back whenever it is convenient? The details of how he would fail his trust -- turning on a superior, harming a guest -- are less important than the fatal example he would give by betraying trust in any way at all.

Macbeth has high regard for King Duncan. He knows that Duncan has been an excellent king, wise and fair, and that the people love him. Macbeth says that "tears shall drown the wind" when Duncan dies.

Macbeth knows that Duncan trusts him and values his service. Shortly before Duncan is murdered, he had made Macbeth Thane of Cawdor to reward him for his loyalty and bravery in fighting against the forces of the King of Norway that had attacked Scotland. Finally, Duncan is a guest in Macbeth's castle. It is Macbeth's responsibility as Duncan's host to protect him, not kill him. 

Just to take a bit of a different angle concerning Shakespeare's Macbeth, first, no evidence really exists in the play that Macbeth was nurturing evil, corrupt thoughts before he hears the witches' predictions.  It seems logical, but it's difficult to definitively declare that.  Second, if he was, then Shakespeare doesn't show a fair mind being corrupted subtly and gradually over time, since Macbeth was already corrupted when the play opens.  You might be able to argue one or the other, but you can't logically argue both.

That said, again, looking at this issue from a different angle, the witches predict only that Macbeth will be king, not that he should assassinate Duncan to get the crown.  Macbeth creates that idea on his own. 

Once Duncan names Malcolm as his heir, however, Macbeth has little choice, if he ever wants to be king.  Malcolm stays behind the lines during the opening battle with his father while Macbeth does the fighting--Macbeth could hardly expect to outlive Malcolm, and even if he does, there's always Donaldbain. 

If Macbeth is obsessively ambitious, he has little choice but to kill the present monarch. 

Obviously, he shouldn't.  Murder has physical, emotional, and spiritual consequences.  The murder of a benevolent King who was also family?  Even worse.  Macbeth becomes a traitor, a bloodthirsty tyrant, a paranoid sleepless zombie, and a necromancer, not to mention a headless corpse.

Before the play, all was well.  Macbeth won victory and favor with the King.  But a combination of ambition, carefully planted seeds by the witches and his wife, and Duncan's naming Malcolm Prince of Cumberland cause Macbeth to take on a Machiavellian quest to commit regicide and fratricide (the two worst crimes imaginable for a Thane).

The murder of the King disturbs social and natural order: witches become advisors, horses eat each other, the earth trembles, fair becomes foul, the king murders his best friend, women and children, the royal family sleepwalk and talk to the dead.  All in all, a hell on earth and a toppling of the Great Chain of Being.

Macbeth, after talking with his wife, Lady Macbeth, seemed to be 100% in on her plan to kill Duncan and take his position in Scotland as King and Queen.  However, while the others were eating, Macbeth is in his chambers thinking.  He has many doubts and talks himself out of the plan stating that Duncan was there "in double trust."  Duncan is Macbeth's relative (blood is thicker than water, and a relative just does not kill a member of his own family...it trickles down from the Anglo Saxon idea that murdering a brother or member of your blood line is the most horrible of sins), and Duncan is a guest in his own home.  Macbeth is a warrior and subject of Duncan, to whom Macbeth has sworn his loyalty.  He should be protecting Duncan for all three of these reasons, not planning on killing him.  In addition, Duncan has given Macbeth lots of attention, praise, and honors for Macbeth's performance in the war against the Norwegians who were led by MacDonwald and the previous Thane of Cawdor. 

So, Macbeth knows that he should be loyal, loving, and willing to risk his own life to protect this man.  He decides not to go through with the murder, and even tells his wife that there will be no more talk of it.  However, Lady Macbeth is an ambitious one who works magic with words on her husband.  She changes his mind in Act I, and the act is committed in early Act II.

Although I disagree that this is Macbeth's "opening soliloquy," I know the precise moment to which you are referring:  Act 1, Scene 7.  (It's one of my favorite pieces of evidence, actually, that it's Lady Macbeth and not Macbeth himself that is the true evil force behind this foul murder.)  In short, Macbeth gives four precise reasons for not wanting to kill Duncan, ... or "why it would be wrong" as you say.

First, Macbeth is afraid of eternal fire.  Macbeth says that "We'd jump the life to come" and they would "plague the inventor."  Well, yes, I would say.  No one ever wants to "plague the inventor"!  Murder is certainly a most foul way to do it, too!  It's interesting, in my opinion, that this is the first reason Macbeth gives.  It would certainly be enough for me to stop dead in my tracks!

Second, Macbeth is afraid of being killed himself when he does become king.  (How ironic!)  Look at this line:

This even-handed justice / Commends th' ingredients of our poisoned chalice / To our own lips.

Macbeth, then, fears drinking from this same "poisoned chalice."  It's just a fancy way of saying that fate will, most likely, come around.  Death will be waiting for Macbeth if he does this, king or not.

Third, Macbeth will be nixing something very important:  loyalty.

He's here in double trust: / First, as I am his kinsman and his subject, / Strong  both against the deed; then , as his host, / Who should against his murderer shut the door, / Not bear the knife myself.

Macbeth speaks of Duncan here.  Duncan is trusting Macbeth in two very important ways.  First, as kinsman/subject, Macbeth should always serve the king by protecting him.  Secondly, as host, Macbeth should do the same.

Fourth, Duncan is a good king; therefore, he shouldn't be killed.

Besides, this Duncan / Hath borne his faculties so meek, hath been / So clear in his great office, / that his virtues / Will plead like angels trumpet-tongued against/ The deep damnation of his taking-off.

It's pretty amazing, actually, how Macbeth enumerates the reasons why Duncan is a good king.  He's humble, "clear in office," and virtuous.  By the end of this soliloquy, one wonders why Macbeth is even listening to his foul wife!  I'll admit, too, that each time I read it, I root for Macbeth to reveal the plot to Duncan and divorce his wife.  I'm afraid I'm always disappointed.

I assume you're speaking of Macbeth's first speech in Act I scene vii.  Here is a soliloquy in which Macbeth is reconsidering their plan to murder Duncan. He lists several reasons why he should not kill Duncan.  They include the following:

He's here in double trust;
First, as I am his kinsman and his subject,Strong both against the deed; then, as his host,Who should against his murderer shut the door,Not bear the knife myself.

Macbeth says he's not only related to him (reason number one), but he's also his host (reason number two) and should be planning how he can best protect him, not how he could kill him. Reason number three is that Duncan is a good man who does not deserve such a fate.  He praises Duncan's meekness and his virtues, noting that all of nature will mourn him--and damn or condemn the murderer--with a flood of tears when he's gone.

Finally, Macbeth finished lamely with his only motivation for killing Duncan:

I have no spur
To prick the sides of my intent, but onlyVaulting ambition, which o'erleaps itselfAnd falls on the other.

Simply put, there are many reasons NOT to kill Duncan and only reason TO do so--his own selfish ambition. Shakespeare's Macbeth is the story of this very concern--Macbeth's arrogance and ambition in the face of reason and right.

One of Macbeth’s arguments is what would happen if they should fail, and Lady Macbeth counters the argument by saying that if he has courage they can’t fail.

Lady Macbeth gives a brutally descriptive account of how she is tough, in order to force Macbeth to be tougher.  His response is to ask: “If we should fail?”  Lady Macbeth does not react positively to that!

We fail?

But screw your courage to the sticking-place,

And we'll not fail. (Act 1, Scene 7)

Basically, Lady Macbeth argues that there is no way they can fail because they have a perfect plan.  She wants him to kill Duncan, leave the knife near the guards after she gets them drunk so that they can’t protect him.

Lady Macbeth shows her reasons for not having faith in her husband, and he seems to also not have much faith in himself.  Macbeth does not really want to kill Duncan, but he does want to be king.  His wife realizes that in order for him to do the deed, she will need to push and prod him along because he doesn't have the guts or motivation to do it himself.  So she both cajoles and insults him until she gets him to do it.